The instant invention is in the field of methods and apparatus for the measurement of viscoelastic properties of molten thermoplastic polymers. More specifically, the instant invention relates to methods and apparatus for the determination of melt elasticity.
As stated in Wagner and Bernnat, J. Rheol. 42(4), Jul./Aug. 1998, p917-928, the melt elasticity of polymer melts is of great importance for many polymer processes like fiber spinning, film blowing, blow molding, high-speed coating, and sheet casting. A measure of melt elasticity can be made by the use of an “extension diagram,” where the drawdown force needed for elongation of an extruded strand of melted polymer is measured as a function of a increasing drawdown speed. For this purpose, a tensile tester, the so-called “Rheotens,” was developed (e.g., see Meissner, Rheol. Acta 10, 1971, p230-242). The Rheotens test is readily performed, shows excellent reproducibility, and models industrial polymer processes like fiber spinning or film casting. Therefore, the Rheotens test has found widespread application.
In a Rheotens test a strand of polymer melt extruded by a polymer melt viscometer is elongated under the action of rotating wheels which have gripped the meltstrand when the velocity of the wheels is larger than the output velocity of the strand. Normally the wheels are accelerated till the strand breaks or the maximum rotational speed of the wheels is obtained. A direct conversion of the tensile-force/drawdown-speed diagram into a relation between elongational viscosity and elongation rate is not possible. However, a considerable simplification in the analysis of constant force extension resulted from the discovery of “Rheotens mastercurves” for thermorheologically simple polymer melts (Wagner, et al., Polym. Eng. Sci. 36, 1996, p925-935). Rheotens mastercurves provide a basis for a direct and quantitative comparison of the elasticity of polymer melts under processing conditions.
Surprisingly, even for Rheotens experiments performed at different extrusion pressures (termed extrusion “stress” in the art), Rheotens mastercurves can be found, if force and draw ratio are scaled appropriately as reported by Wagner, et al. Such mastercurves, which represent mastercurves of mastercurves, are termed “Rheotens supermastercurves.” Wagner and Bernnat showed that the concept of Rheotens mastercurves could be generalized to experiments with extrusion dies and spinlines of different length, and that information on the elongational viscosity of polymer melts could be extracted from Rheotens mastercurves by use of an analytic rheological model.
Instruments for performing the Rheotens test are commercially available from Goettfert Inc., Parkway Rock Hill, S.C. However, the commercially available instrument for performing the Rheotens test is relatively expensive because it employs a polymer viscometer to generate the meltstrand. Furthermore, such instruments are considered by the art to be more useful in a research laboratory than in a quality control laboratory of a polymer production facility due to the expense of the instrument and the degree of skill needed to operate the instrument. Thus, there is a need for the development of a less expensive and more readily operated instrument for the determination of melt elasticity of a thermoplastic polymer.